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Minimum Rotor Thickness

W

waehrik

Guest
I have a 75 R90/6 on which the single drilled stainless rotor is pulsing when using it and I measured the disk to be warped by about 0.005". I can't find a minimum thickness spec on the rotor itself though, the aluminum had a nice white corroded finish that even when removed has obscured any markings that were once present on the hub. The rotor now measures 5.3 mm.

Does anyone know what the minimum thickness spec is of these drilled stainless rotors?

Thanks!
 
Last edited:
Check on the inboard side of the disc carrier. Stock min is 4.6mm or 0.181"
 
There is no such thing as a warped rotor. It is a myth handed down through generations of shade tree mechanics.

What you are experiencing (and measuring) is uneven transfer of the brake pad material causing alternating low "valleys" and high "hills" on the surface of the rotor. Caused by light braking.

Generally speaking, stainless alloy rotors cannot be turned except by a VERY specialized facility - and then the cost becomes prohibitive since the additional life you gain is limited.

As an alternative you may want to consider scrubbing both sides of the rotor with a sanding block with continuously finer grades of garnet paper. The deposits - known as cementite inclusions - are invisible and as hard as a ceramic. Sometimes - depending on the thickness of the depositions - you can eliminate them. They usually return, (once again due to light braking) and you'll still need to buy new pads once you're done. Make sure to clean EACH hole completely as well. Wipe everything down with acetone and a white rag. You'll know you're done when you have no more dark spots on your rag after wiping. It is a lot of work.

Although it is a chunk of change the easy way is just bite the bullet and buy a new rotor and pads - and bed them in properly. I would AVOID after market pads. I have had major issues with them since they are not machined to fine enough tolerances and can have sloppily applied cosmetic powder coating which will not let them mount perfectly square in the caliper.

The attached picture shows a finished rotor after "sanding" with 600 grit garnet - ready to be returned to service. Notice the cross hatches which will gradually disappear over several hundred miles as the new pad material fills them in.
 

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I'm curious why rotors cannot be warped. My test setup was a Mitutoyo digital drop gage rigidly mounted to the left for slider with the tip resting on the rotor. As the wheel rotated I could visibly see the rotor's deflection in the caliper and the corresponding readout on the gage. From high to low was 0.005". Both sides deflected, like the rotor was bent.

There does seem to be a hard spot on the rotor too, as evidenced by a dark splotch - that would certainly cause the sinusoidal wear pattern.

Since it seems that I'd have to take off too much material to eliminate the wear pattern, I'm better off buying a new rotor and pads. I've heard good things about getting both from EBC here - have you had a different experience?

RonKMiller said:
There is no such thing as a warped rotor. It is a myth handed down through generations of shade tree mechanics.

What you are experiencing (and measuring) is uneven transfer of the brake pad material causing alternating low "valleys" and high "hills" on the surface of the rotor. Caused by light braking.

Generally speaking, stainless alloy rotors cannot be turned except by a VERY specialized facility - and then the cost becomes prohibitive since the additional life you gain is limited.

As an alternative you may want to consider scrubbing both sides of the rotor with a sanding block with continuously finer grades of garnet paper. The deposits - known as cementite inclusions - are invisible and as hard as a ceramic. Sometimes - depending on the thickness of the depositions - you can eliminate them. They usually return, (once again due to light braking) and you'll still need to buy new pads once you're done. Make sure to clean EACH hole completely as well. Wipe everything down with acetone and a white rag. You'll know you're done when you have no more dark spots on your rag after wiping. It is a lot of work.

Although it is a chunk of change the easy way is just bite the bullet and buy a new rotor and pads - and bed them in properly. I would AVOID after market pads. I have had major issues with them since they are not machined to fine enough tolerances and can have sloppily applied cosmetic powder coating which will not let them mount perfectly square in the caliper.

The attached picture shows a finished rotor after "sanding" with 600 grit garnet - ready to be returned to service. Notice the cross hatches which will gradually disappear over several hundred miles as the new pad material fills them in.
Click to expand...
 
I beg to differ. I have cut many hundreds of disc brake rotors. Maybe we should call it excessive run out? A Formula Ford, that I used to crew on was a really good example. The rear in-board mounted rotors had enough warpage or runout to push the pistons back, visible to naked eye.
 
Whether they warp in use due to heat and uneven cooling is an open question, and depends on too many variables to cover here. But they surely can wobble when a tire change or other traumatic event bends one. While this is only one of a few possibilities that cause pulsing, it is a very common one.
 
Ah, I love a good friendly debate! ...but only based on facts and not myth. :brow It's been pretty boring around here lately at Airhead Central anyway.

So, here's some good reading to start with:

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

But 'ya gotta' promise me you'll read the whole article...

If you don't believe what you've read just be prepared to describe the exact mechanical or chemical changes (besides deformation caused by gross neglect or an accident) that could cause a rotor to "warp".

I'm just a lowly chemist by training :deal - but I can usually hang in there with the engineers and physicists - for a while. :bow

In the meantime I'm gonna go grab a Turkey leg. :eat

Hopefully the L-tryptophan (which is of course another myth...) won't kick in for a while. :D
 

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I've heard good things about getting both from EBC here - have you had a different experience?





I can't comment on the EBC's since I've never used them.

I bought some brand new Lucas pads (TRW) that were recommended by a well known vendor instead of BMW OEM's. What a mess. Sometimes I NEVER learn...

Lucas. What WAS I thinking? :dunno:scratch
 
waehrik said:
I've heard good things about getting both from EBC here - have you had a different experience?
Click to expand...

On my '82 R100RS I put EBC Pro-lite rotors and pads (I think they were double H). The rotors look so good! I have never been able to get them to stop pulsing during heavy braking and they are noisy, sound crunchy when really leaning into them. I need to try the garnet scrubbing that RonK suggests.
On the other hand, I put some slightly used EBC Pro-lite rotors on my R100RT but kept the stock pads (lazy and cheap). That set-up is the best. No complaints there.
 
I put EBC ProLite floating rotors and EBC pads on my /7 (dual disc conversion). I like them a lot. I'd do it again if I had another airhead.
 
I replaced the BMW rotors 13,000 miles ago with EBC Prolite rotors and EBC FA18 organic pads. I am no less than satisfied with them and overall braking performance. They are good looking and an easy bolt-up installation.
 
RonKMiller said:
Ah, I love a good friendly debate! ...but only based on facts and not myth. :brow It's been pretty boring around here lately at Airhead Central anyway.

So, here's some good reading to start with:

http://www.stoptech.com/tech_info/wp_warped_brakedisk.shtml

But 'ya gotta' promise me you'll read the whole article...

If you don't believe what you've read just be prepared to describe the exact mechanical or chemical changes (besides deformation caused by gross neglect or an accident) that could cause a rotor to "warp".
Click to expand...

Ron,

I'm not going to argue with you, and the article was good about cars - but surely you have noticed the big difference between the construction of car disks, and especially race car disks, and those on our BMW motorycles. (See Myth #2) Both of our cars/trucks (a Saturn SC1 coupe and a Ford Explorer) have thick cast disks that look like two disks with cooling vanes cast between them. All of our BMWs have thin stainless steel disks that look like thick sheet metal in comparison.

Certainly, the warpage, bendage, breakage variations of these two different construction types can't go unnoticed. I say - good article but apples and oranges. He did note variations in thickness measurements, but didn't address axial runnout (wobble) because that almost never happens with cast iron vaned disks, which was what he was writing about. Which cannot be said for stainless steel motorcycle disks.
 
Thanks everyone for the great info!

I'll try sanding down the disc first before paying the big money for a new rotor, but unfortunately I'm not very optimistic about it working. The disc is definitely warped (bent) and likely just needs to be replaced. It's not terribly surprising though, the bike had been sitting for the better part of 30 years when I picked it up.

I think I'll go with the EBC Prolite rotors and organic pads - I've had excellent results with organics in the past and like how they're more gentle on the rotor (so hopefully this won't happen again)
 
old thread eh?

I just bought one of these hones:

https://www.brushresearch.com/brushes.php?c1=6

It may be crap, but was recommended by several buddies.
I have some pulsation on my front rotor and will be using this hone to reapply a nice NDSF
(Non-Directional Surface Finish) and see if it fixes things up ... if not new rotor time.

I will post back with the results ... if I can remember... :blush
 
This may sound really crude, but after sanding down that rotor, if it still is out of true, [ got my flame suit on here ] I would try and whack it with something, like a Oak 4x4 or dense rubber mallet, especially if cost is a issue. If got out of true somehow and who knows you might get that .0005 back, just saying..
I would turn it and mark the "high" point with a magic marker, really find "the spot".
Like Paul said, those things are really thin, close to thick sheetmetal :thumb
 
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